You'd expect this cloudy blue glass to throw a blue light onto its surroundings. The light it throws, though, is clearly a bright orange-yellow. Can you guess why?

Put a red vase up to a window and anything it its light will be covered in red. A blue piece of plastic should work the same. And yet it's clear to see that this stone throws orange light behind it. How can a light change from blue to orange? For the same reason you can see a beam of light in a dusty room, or a laser through a cloudy glass of water solution. The Tyndall Effect shines through.

The Tyndall Effect, or Tyndall Scattering, is named for John Tyndall, the man who studied it in the nineteenth century. He studied it with much more modest tools, simple beams of light and glasses of water with powder in them. He was working with colloids, liquids with microscopic substances dispersed evenly through them. These microscopic substances didn't dissolve, like salt in water. They just floated, more or less in perfect suspension in the solution. Make the colloid weak diffuse enough and the particles aren't even visible. Until you shine a light on them. All those tiny particles catch the light and reflect it in many different directions, including back at the onlooker. (For those of you who are fond of spy movies, the Tyndall Effect is also happening when someone sprays aerosol across a room and suddenly you can see laser beams criss-crossing the thing.)

Of course, the light that gets bounced off the particles gets diverted from its final destination. That, in most cases, will only make the light at that final destination a little more dim. However, sometimes the particles are of the size that they only catch and reflect some of the light. For example, they might get the blue light, but the red light wavelengths are too big and just move past them. In this case, they reflect blue every which way, including back towards the onlooker, and so not much blue gets to its final destination. Take the blue out of white light, and you'll get a yellowy-orange light. That's what we're seeing here. And yes, this is the kind of thing that makes the sky blue but the light hits the Earth yellow.